A hydrogel is a network of hydrophilic polymer chains in which water is the dispersion medium. The hydrogel can contain a large amount of water in the structure thereof because it is not dissolved, but swelled in water, and thus it possesses a degree of flexibility very similar to natural tissue.
The hydrogel has been used in the medical and pharmaceutical field, such as the fields of tissue engineering, cell culture, sustained-release drug delivery systems, biosensors, soft lenses, medical electrodes, and the like, due to its unique hydrophilicity and flexibility, and has also been applied in the cosmetic field to prepare a mask pack for delivering various advantageous effects including moisturizing, nourishing, wrinkle improvement, whitening, and the like to skin.
In the case of an application in a mask pack, research on and the development of a method of preparing a sheet type hydrogel are being actively conducted. Conventionally, when a hydrogel is prepared in the form of a sheet, the shape thereof cannot be maintained only by a hydrogel, and it has poor mechanical properties such as low strength or low hardness. Therefore, it is common to use a hydrogel in addition to a support such as a synthetic resin, a non-woven fabric, a mesh, or a net. However, a mask pack using a support such as a non-woven fabric or the like as a base is easily dried and has poor adhesion to skin, and a hydrogel is often detached from the support. In addition, since a support such as a non-woven fabric and the like is prepared through a chemical process, when a mask pack including such a support as a base is applied to skin, skin trouble may occur, and thus many safety evaluations and reviews are required.
As a method for solving these problems, Patent Document 1 (Korean Laid-Open Patent Publication No. 10-2013-0036543) discloses a hydrogel composition for a base of a mask capable of maintaining the shape thereof without a support and a method of preparing a hydrogel using the same. Specifically, the hydrogel composition includes 0.1 to 10 wt % of a crosslinking agent, 0.2 to 6 wt % of a gelling polymer, 0.5 to 20 wt % of a polyhydric alcohol, and 70 to 90 wt % of purified water, and the method of preparing a hydrogel includes (i) preparing an aqueous solution by adding a crosslinking agent to purified water at room temperature and then stirring the mixture at 40 to 85° C.; (ii) preparing a hydrogel composition by dissolving a gelling polymer in a polyhydric alcohol at room temperature, then adding a resulting substance to the aqueous solution, and stirring the mixture at 40 to 80° C.; (iii) compression coating the hydrogel composition so as to have a thickness of 0.5 to 2 mm; (iv) preparing a hydrogel by cooling the hydrogel composition layer prepared through compression coating at room temperature; and (v) thermally treating the hydrogel thus cooled at 40 to 85° C. for 12 to 36 hours.
According to the step (ii), a liquid hydrogel composition is prepared through a heating and stirring process. In this process, a large amount of air bubbles may be generated in the liquid hydrogel composition, and if the air bubbles thus generated are not removed, a resulting hydrogel sheet may be adversely affected. Specifically, when the liquid hydrogel composition is solidified into a hydrogel sheet while still containing a large number of air bubbles, an opaque and aesthetically unappealing hydrogel sheet is prepared. Not only that, since air bubbles trapped inside the solidified hydrogel prevent the hydrogel from having a three-dimensional network structure, a gel sheet with poor properties such as low gel strength is prepared.